15 1 Bacteria Although two major groups of prokaryotes are now recognized we will study the bacteria as a represente vo prokaryote Most bacteria are saprotrophic meaning that they send out digestive enzymes into the environm and thereafter take up the resulting nutrient molecules Saprotrophic organisms are decomposers that play utilizin major role in ecosystems by digesting the remains of deal organs and returning inorganic nutrients t Photosynthetic organisms Some bacteria are parasitic and care aces such as strep throat or gangrer inorganic molecules Cyanobacteria are always photosynthetic They contain chlorophyll but the green color is often masked by other pigments In fact some cyanobacteria are red brown or even black Several techniques are used to identify bacteria three methods are demonstrated in this laboratory Ciram colony shape of cell Gram Stain Most bacterial cells are protected by a cell wall that contains a unique molecule called peptidoglycan Bacteria are commonly differentiated by using the Gram stain procedure which distinguishes bacteria with a thick layer of peptidoglycan Gram positive from those that have a thin layer of peptidoglycan Gram negative in their cell wall Gram positive bacteria retain a crystal violet iodine complex and stain blue purple whereas Gram negative bacteria decolorize and counterstain red pink with safranin Fig 15 2 Figure 15 2 Generalized structure of a bacterium a Gram positive cells have a thick layer of peptidoglycan b Gram negative cells have a very thin layer of peptidoglycan c This difference causes Gram positive cells to stain purple and Gram negative cells to stain reddish pink ja ASM Science Source peptidoglycan plasma membrane a Gram positive cell outer membrane peptidoglycan plasma membrane b Gram negative cell c Micrograph of Gram stained bacteria Observation Gram Staining Examine prepared slides under high power Note the color differences Fig 15 2c Study the directions for Gram staining on the next page Why do the Gram positiva gall CO